Extra-heavy or bituminous crude oils, also referred to in the present application as heavy crudes or bitumens, represent considerable resources that are and will be increasingly developed. However, these crudes have physical properties, notably very high viscosity and density, which make their extraction, production, transport and processing difficult.
Such crudes can therefore not be extracted using conventional methods.
Extraction methods specific to this type of crude have thus been developed. One, suited for surface or shallow reservoirs, referred to as mining extraction method, consists in mixing sand with the crude to be extracted and in extracting the mixture of sand and crude mechanically. This mixture is then washed, separated and the lighter cuts are then upgraded.
For deeper reservoirs, this method is unsuitable and on-site production has to be assisted in order to make them mobile, i.e. decrease their viscosity so as to make extraction possible.
In order to decrease the viscosity, the ground is heated by steam injection and the crude thus made mobile can be extracted. These method's, referred to as steam-assisted gravity drainage or SAGD, or cyclic steam stimulation or CSS, are described in documents U.S. Pat. No. 4,344,485, U.S. Pat. No. 4,850,429 and U.S. Pat. No. 5,318,124. Although widely used, these methods involve the major drawback of consuming very large amounts of natural gas required for producing injected steam. Their profitability therefore greatly depends on the price of natural gas.
Besides, the crudes thus extracted exhibit high asphaltene and heteroatom (S, N, O, V, Ni, etc.) contents. They must therefore be processed in order to have synthetic crudes of satisfactory quality, i.e. with a viscosity and a density allowing pipeline transportation, and a low proportion of sulfur and other heteroatoms. The upgrading stages also consume large proportions of natural gas, which is notably necessary for hydrogen production through steam reforming of natural gas or methane.
In order to minimize this dependence on natural gas, U.S. Pat. No. 4,399,314 describes a method wherein a bitumen from a bituminous sand is subjected to hydroconversion, the hydroconversion residue is gasified with oxygen in order to produce a synthesis gas from which hydrogen is produced for the hydroconversion stage.
U.S. Pat. No. 6,357,526 provides a deasphalting method for recovering a deasphalted crude that constitutes the synthetic crude and the asphalt is burned to generate steam that is used in the SAGD extraction method. However, the synthetic crude obtained is not of good quality because it still contains many contaminants such as sulfur, nitrogen and metals.
There is therefore a real need for a method of preparing a synthetic crude from an extra-heavy or bituminous crude reservoir, allowing to obtain a quality synthetic crude whose dependence on the price of natural gas is decreased or even cancelled out.
Patent FR-2,887,557 provides a sequence of methods including the extraction and processing stages, combustion and/or gasification of the conversion residue allowing to generate energy in form of steam or electricity and/or hydrogen, the steam being then used for extraction and the hydrogen for processing by carrying out the stages described below.
This method allows to avoid using natural gas but it requires combustion and gasification of the residue. The combustion of petroleum residues using conventionally known methods is carried out by direct contact with air, which results in significant CO2 emissions since the fumes generally contain of the order of 10 to 15% (by volume) of CO2 diluted in the nitrogen in the air. It is also possible to use the new oxycombustion techniques to burn the residue. In fact, oxycombustion allows to produce CO2-rich fumes (content above 90 vol. %) by contacting the residue only with oxygen that has been separated from air in an ASU (Air Separation Unit) beforehand. CO2 capture is then facilitated. Compression of the fumes allows to directly consider reinjecting the fumes into a storage site. Unfortunately, an ASU requires a very high investment (above 100 million euros for a 350 t/h O2 unit) and a high energy consumption. The result thereof is a decrease in the combustion cycle efficiency, from typically 47% to 50% for a conventional air combustion unit to 39% to 42% for a oxycombustion unit because of the energy consumption induced by the air separation unit. Gasification of the residue also requires an air separation unit (ASU) for introducing the purified oxygen. The same advantages and drawbacks are observed: the CO2 can be readily captured in nitrogen-free fumes. However, the energy consumption induced by air separation is high.
Direct residue combustion by contacting a metallic oxide with the hydrocarbon in a chemical looping cycle is described in patent application FR-08/02,450.
It is then possible to directly produce combustion fumes containing more than 90 vol. % CO2 without affecting the combustion cycle efficiency that remains close to 47 to 50%. In fact, the oxygen directly comes from the metallic oxide that circulates continuously between a hydrocarbon combustion zone and a reduction zone with contacting with air wherein the metal reduced after combustion is reoxidized. In the combustion zone, the fumes essentially contain CO2 since it is not necessary to use air. In the reduction zone, the gaseous effluents consist of depleted air free of the oxygen captured by the metallic oxides. Circulation between the two zones occurs by means of conventional fluidized bed techniques. In order to carry out satisfactory combustion, it is desirable to operate the combustion and oxidation zones at temperatures ranging between 800° C. and 1100° C., preferably between 900° C. and 1000° C.
It is also possible to produce a mixture (CO, H2) and therefore hydrogen by limiting the metallic oxide circulation between the two enclosures and by feeding steam into the combustion zone. It is thus possible, by means of a chemical looping cycle, to produce energy without significantly decreasing the energy efficiency of the combustion while having fumes containing at least 90 vol. % CO2, or to produce synthesis gas, again with an improved energy efficiency.
We have discovered that chemical looping combustion can be advantageously used for upgrading the unconverted residue from an enhanced heavy crude recovery process and improving the energy efficiency of the method, while allowing capture of the CO2 emitted.